The size depends on the typeface, but there are multiple variables involved. If you want to keep it simple, an easy starting point it to remember that for optimal readability, aim for 40–60 characters per line for your body text. From there you can increase the size/weight of the heads. There are no perfect answers here—setting type is an art and takes practice. A lot of us collect publications that we like, and then use them for inspiration when it is time to design our own publications. There are also some excellent books out there: